Process for making insulating materials



Patented July 2, 1940 UNITED STATES PROCESS FOR MAKING INSULATDIG MATERIALS Willard H. Bennett, Columbus, Ohio No Drawing. Application July 8, 1937,

Serial No. 152,617 J 5 Claims. (01.106-12) My invention relates in general to the process for making an insulating material and more par ticularly to an insulating material having an extremely porous consistency.

An object of my invention is to provide a medium which will withstand large electrical differences of potential.

Another object of my invention is to provide a material which will withstand in vacuum more than 400,000 volts per five millimeters of thickness of material.

Another object of my invention is the provision of a material which will withstand bombardment in a vacuated tube by electrically charged particles having a high energy and thus possessing'a high velocity.

Another object of my invention is the-provision of a material which will withstand in vacuum bombardment by electrical particles having a higher energy than that required to rupture glass, quartz, or porcelain.

A still further object of my invention is the provision of an insulating body which in itself is non-vitreous in order that electric current cannot pass through the bodyat high voltages and puncture the body.

Another object of my invention is the provision of an insulating material which can be fitted over electrodes in vacuum tubes in order to eliminate open evacuated volumes next to metal surfaces at which high negative electric fields exist.

Another object of my invention is the provision of a material having high porosity which permits rapid evacuation and elimination of gas which may accompany the initial application of high voltage due to local electrical disturbances such as sparks or the bombardmentby high velocity electrical particles.

Another object of my inventionis the provision of a material having high porosity which gives a mechanical toughness and reduces the risk of brittle fracture due to electrical stresses. 4

Another object of my invention is the provision of a material having high porosity which in.- creases the volume resistivity over what it would be if the body were not so porous.

A still further object of my invention is to provide a material which may be used in evacuated tubes as a mechanical support for electrodes or other metal surfaces where the mechanical support must be an insulating material that is relatively free from objectionable vapors.

Another object of my invention is the provision .of an insulating material which may be used in evacuated tubes as a mechanical support for electrodes or other metal surfaces, which material itself doesnot giveofi excessive "vapors.

Another object oi my invention is to provide a crucible material which is free from those ele ments that are objectionable in certain electrical '5 preparations of metals,

Another object of my invention is to provide a heat resisting material having a low heat con ductivity.

Another object of my invention is to provide a crucible material used in certain electrical preparations, in which the elements of the crucible material does not have a chance to dissolve because ofthe low heat conductivity.

Another object of my invention is to provide a 15 material which lacks theose elements or oxides which vitrify at intermediate temperatures.

, Another object of my'invention is the provision of a material having an' extremely porous struc- 2o ture.

Anotherobject of my invention is the process of forming an extremely porous structure by heating the elementsthereof through the stage of condensation and to substantially-the point where sintering begins.

Another object of my invention is the process of producing a material of extremely porous structure by'restricting the firing temperature during its formation.

Anpthr'qbject Of y nvention is to provide an insulating material which may be worked by a cutting tool.

Another object of my invention is to provide an insulating material which strength in vacuum. I Another object of my invention is, to provide a material which prevents the formation of cold emission streams, sometimes referred toas cold cathode discharges.

Other objects and a fuller understanding of 40 my invention may be had by referring to the following description and claims.

In the embodiment of my invention, the principal constituent may be diatomaceous earth which in its natural dry state isv non-cohesive.

Diatomaceous earth may sometimes be referred to as kiesel guhr and comprises an earthy deposit or sediment in water and is loose or porous diatomite, and abounds in diatomswhich com I prises any of the microscopic unicellular or colonial algae constituting the class Bacillarz'eae. Diatoms have a silicified cell Wall which persists as a-skeleton after death and forms kieselguhr.

Diatomaceous earth may sometimes be referred to 5 has a high dielectric 35 as amorphous silica each particle of which itself is very porous.

The particles of the diatomaceous earth, which are non-cohesive, are bound together during the process of preparation by a binding refractory material. In the practice of my invention, I find that kaolin or clay may be used. Of the clay group, English ball clay appears to be preferable. The term English ball clay is used among ceramists and designates a definite product in the trade. The applicant finds that English ball clay comprising the following chemical composition and physical characteristics, may be suitable "in the embodiment of his invention:

The chemical analysis of the English ball :clay may be as follows:

'Per cent Silica 57.28 Alumina 26.38 Oxide of iron -i 0.79 Lime 0.08 Magnesia 0.45 Potash 2.57 Soda 0147 Oxide of titanium 1.60 Sulphuric acid 0.03 Combined water and organic matter 7.25 Moisture 3.10

The physical analysis of the English ball clay may be as follows:

Per cent res. on 120 mesh h... 0.00 Extrus. length inches 6.4 Per cent water plas -F. 352 Per cent vol. dryg shr--" 25.2 Per cent shr. water 19.2 Per cent pore water 16.0 Per cent lin. dryg shr 7.7 Bulk spec. grav s n 1.749 App. spec. grav 2.647 Per cent dry app. porous 33.9 P. C. E 30-31 (a) v32A Modulus of rupture (b) 381 Slaking time min 22 pH in distilled water 6.56

It is to be understood that I do not intend to limit my invention to diatomaceous earth and English ball clay, because other substances having similar structures and properties, which may be called by some other name, may be used.

In preparing my insulating material a high grade of dry diatomaceous-earth is first stirred in distilled water. The amount of water should be such that the consistency is made syrupy and not mushy. At no time during the mixing may the consistency become mushy. It is found that 4 parts by weight of dry diatomaceous earth stirred in approximately 11 parts by weight of distilled water will give a syrupy consistency. This ratio may be varied between the limits of a thin to a thick syrupy consistency, but must not be extended to include a mushy consistency.

After the syrupy consistency of the diatomaceous earth and the water is obtained, the next step is to stir in slowly finely divided English ball clay. still maintaining the syrupy consistency. The mechanical strength of the finished insulating material may be varied by governing the amount of the English ball clay added. It is found that a relatively strong mechanical insulating material may be formed by varying the English ball clay between the limits of one-half to one part by weight, to four parts by weight of diatomaceous earth.

After the diatomaceous earth and the English ball clay is completely merged in a syrupy consistency, which may now be designated as a slip, it may remain in suspension for approximately a week to determine whether or not the mixture will retain its syrupy consistency, which may be lost by the settling of any sandy or solid residue. This is primarily to test whether or not the proper proportions have been used for making the slip. However, in actual production methods the test may be eliminated because when once it is determined that the proper proportions have been attained it is not necessary to make further tests because uniformity may be obtained if the same grade of ingredients are used throughout the manufacturing process. The distilled wateris used to avoid the presence of electrolyte which tends to throw down a hard residue from suspension.

In the manufacture, the slip having the syrupy consistency only is used The slip may be cast into any suitable form or shape to meet the requirements of the eventual use of the insulating material. The castingof the slip may be done in plaster molds, and it is preferable that the molds be dampened by filling them. with water for about 10 minutes beforethe slip is poured into the plaster molds. While I have suggested the use of plaster molds, it is to be understood that any other suitable molds may be employed so long as the same results are obtained. The slip is poured slowly into the molds as it slowly forms, after which the excess may be poured out as is usually done in the casting of porcelain.

The water in the slip is absorbed by the plaster molds and the forming of the slip in a substantially solid mass may take from an hour for a casting with a thickness of a few millimeters and several days for a casting with a thickness of 10 centimeters; After solidification, the castings may be left in the molds for a few days, keeping the pouring holes covered in order to maintain a uniform humidity inside the molds. The cast-,

ings may be removed from the molds after it is H sufficiently rigid to handle, and allowed to completely dry.

After the castings have completely dried, they may then be fired slowly up to approximately 1000 degrees centigrade. inserted in the furnace, it is substantially at room temperature and the temperature is gradually increased up to approximately 1000 degrees centigrade. The time for this gradual'increase in temperature may vary from 2 to 10 hours. The firing is continued through the stage of condensation and is stopped just when sintering begins at approximately 1000 degrees centigrade, thus avoiding the loss of any appreciable degree of porosity which may be available. When stopping the firing at this point some mechanical strength may be lost which would have been attained by a higher firing temperature, but this is not a serious handicap. After the firing has reached the stage of sintering in the neighborhood of 1000 degrees centigrade, the furnace is shut off and i the temperature gradually recedes to room temperature, after which the castings are removed. The castings may then be drilled or worked by a sharp cutting tool, or they may be mounted on a lathe and turned. The methods of working are similar to those of wood turning. In mounting the piece to be worked, in a lathe only a gentle grasp of the piece can be used since it is fragile and will crumble under excessive forces.

The finished piece may be used in several applications. For example, itv finds excellent use in connection with high voltage vacuum tubes. The insulating material may be worked or shaped to fit closely against any or all metal surfaces within high voltage tubes in order to eliminate open evacuated volumes next tothe metal surfaces at which high negative electric fields exist. It has a high dielectric strength and thus prevents the formation of cold emission streams, sometimes referred to as cold cathode discharges. Also, the insulating material itself is non-vitreous in order that electric currents cannot pass through the body at high voltages and puncture the material. The material will withstand more than 400,000 volts per five millimeters of thickness, in a tube in which the pressure is less than one-tenth micron of mercury. The material will withstand bombardment in vacuum by electrical particles having a higher energy than that required to rupture glass, quartz, or porcelain. The material has a high porosity and permits rapid evacuation and elimination of gas which may accompany the initial application of high voltage due to local electrical disturbances such as sparks or the bombardment by high velocity electrical particles. The material increases the volume resistivity over what it would be if the body were not so porous, and may be used in evacuated tubes as a mechanical support for electrodes or other metal surfaces where the mechanical support must be an insulating material that is relatively free from objectionable vapors, that is to say the material itself does not give off excessive vapors. It has a mechanical toughness and reduces the risk of brittle fracture due to electrical stresses.

The material may be used as a crucible in certain preparations of metals, and particularly those prepared by rapid method of heating, as by an electric are or induction. It is free from most of the elements which are objectionable in the preparation of metals. The heat conductivity is extremely low, so that the contents in the crucible may be heated and allowed to cool before the crucible becomes heated to more than a very slight depth. Consequently the elements of the crucible do not have a chance to dissolve in the melt.

The chemical inertness of my material makes it useful in lining vessels to protect the wall of the vessels from corrosive or metallic vapors origihating in the interior of the vessel.

My material is extremely porous. The prim cipal constituent: namely, diatomaceous earth,

7 consists of microscopic particles which in them" selves have a certain degree of porosity. Consequently a body made up of these particles will have inherently a degree of porosity attributable to the said individual particles in addition to the porosity which may be attained through the process of formation of the body. The pores may range in size from one thousandth of a millimeter to smaller sizes, and are many times more in number per unit volume than in any other rigid dielectric substance. It is primarily the large number of pores which give my material its excellent electrical properties.

The material can be fired to a rigid consistency without any appreciable vitrification, because the constituency lacks those elements or oxides which vitrify at intermediate temperatures, and thus a maximum porosity is retained.

Although I have described my invention with a certain degree of particularity, it is understood that the present disclosure has been made only by wayof exampleand that numerous changes in the details of the combination and arrangement of substances maybe resorted to without departing from the spirit and scope of the invention as hereinafter claimed.

I claim as my invention:

1. The process of making a refractory insulating material for a metal body in high voltage vacuum, tubes which comprises the process of making a refractory insulating material which comprises mixing diatomaceous earth, English ball clay and water to form a mixture, drying the mixture to form a substantially rigid mass, and heating the mass in the neighborhood of 1000 centigrade through the stage of condensation to substantially the point where sintering begins, to drive out the residual water and avoid the loss of any appreciable degree of porosity while retaining mechanical toughness to produce a porous non-vitreous structure to withstand in vacuum large electro-static forces produced by large electrical differences of potential.

2. The process of making a refractory insulating material for a metal body in high voltage vacuum tubes which comprises the process of making a refractory insulating material which comprises mixing diatomaceous earth,- kaolin and water to form a mixture, drying the mixture to form a substantially rigid mass, and heating the mass through the stage of condensation to about 1000 centigrade substantially the point where sintering begins, to drive out the residual water and avoid the loss of any appreciable degree of porosity while retaining mechanical toughness to produce a porous non-vitreous structure to withstand in vacuum large electro-static forces produced by large electrical differences of potential.

3. The process of making a refractory insulating material for a metal body in high voltage vacuum tubes which comprises the process of making a refractory insulating material which comprises mixing diatomaceous earth, clay and water to form a mixture, drying the mixture to form a substantially rigid mass, and heating the mass through the stage of condensation to the neighborhood of 1000 centigrade substantially the point where sintering begins, to drive out the residual water and avoid the loss of any appreciable degree of porosity while retaining mechanical toughness to produce a porous nonvitreous structure to withstand in vacuum large electro-static forces produced by large electrical differences of potential.

4. The process of making a refractory insulating material which comprises mixing approximately 4 parts by weight of diatomaceous earth with approximately 11 parts by weight of distilled water to produce a syrupy consistency, adding approximately to 1 part by weight of English ball clay and still maintaining the syrupy consistency, molding the slip formed by the above process to form a substantially rigid mass, and heating the mass through the stage of condensation to about 1000 centigrade the point substantially where sintering begins, to drive out the residual water and avoid the loss of any appreciable degree of porosity While retaining mechanical toughness to produce a porous non-vitreous structure to withstand in vacuum large electrostatic forces produced by large electrical differences of potential.

5. The process of making a refractory insulating material which will in vacuum withstand bombardment by electrical particles having a higher energythan that required to rupture glass; quartz or porcelain comprising the steps of mixing diatomaceous earth, a binding refractory material and water to form a. mixture, drying the mixture to form a substantially rigid mass, heating said mass in the neighborhood of 1000. degrees centigrade through the stage of condensation and to substantially the point where sinterlng begins,

to drive out the residual water and avoid. the. loss of any appreciable degree oI-porosity while retaining mechanical toughness to produce a porous non-vitreous material which will withstand 100,000 volts or more per five millimeter of thickness at a pressure of one-tenth ofa. micron or less.

WILLARD H. BENNETT. 

